Generally, chromatography systems separate analytes of a sample using a separation unit, such as a chromatographic column. For example, a sample containing various analytes, such as chemical compounds, or other sample constituents, dissolved in a solvent solution may be injected into a mobile phase fluid stream with an injection valve, where the mobile phase typically comprises one or more solvents. The sample-containing mobile phase flows through the chromatographic column which selectively retains the analytes from the sample. The analytes from the sample experience a differential retention with the column's stationary phase, e.g., using packing material or sorbent within the chromatographic column, and the relative elution strength of the mobile phase. The separated analytes may then be directed to a detector for detection and analysis, where each of the analytes emerges from the chromatographic column at a different time corresponding to the respective differential retention of that analyte within the chromatographic column. Detection over time results in “peaks” respectively corresponding to the analytes of the sample, where the magnitude of each peak correlates to the amount of the corresponding analytes in the sample. In preparative chromatography systems, the separated sample constituents may be collected by various fraction collection devices.
Typically, the mobile phase is a mixture of solvents provided by corresponding pumping systems. The solvents include at least a strong solvent and a weak solvent referring to the solvents relative elution strength in relation to each other and to the stationary phase being used. The strong solvent favors a partitioning of the sample components into the mobile phase, thus lessening retention, or providing faster transiting of the chromatographic column. The weak solvent favors partitioning of the sample components on the column's stationary phase thus increasing retention, and may serve to moderate the effects of the strong solvent. Attempts are made to balance the mobile phase composition or ratio between the strong and weak solvents in order to provide an acceptable compromise between speed of the chromatography operation and quality of the analytical results.
One type of chromatography system is supercritical fluid chromatography (SFC). SFC with packed columns typically uses an organic solvent, such as methanol, as the strong solvent and highly compressed dense carbon dioxide (CO2) as the weak solvent.
Conventional chromatography systems disrupt the mobile phase to the chromatographic column in order to pressurize the sample loop and/or introduce the sample loop into the mobile phase, including interrupting flow of the mobile phase to the chromatographic column. That is, the mobile phase flow is essentially disconnected from the column, while the sample loop is pressurized with the mobile phase from a pump outlet before the mobile phase flow is essentially rejoined to the column to again perform the chromatography. The switching of an unpressurized element into the flow stream stops the flow of the mobile phase to the column for the period of time required to pump the mobile phase at or near the volume of the sample loop, and to pressurize the same. Although the interruption of mobile phase flow may be relatively short in duration for small volume sample loops (e.g., less than about 10 μl) typically used in analytical SFC, it is still undesirable. Further, for large volume partially filled sample loops (e.g. volumes greater than 20 μl), or very large volumes associated with solid phase extraction (SPE) cartridges or other types of pre-columns (e.g., greater than about 250 μl), the interruption is significant, sometimes lasting for more than 7 seconds, for example, and causing pressure impulse perturbations greater than 50 bar. During this period, the mobile phase pumping system, and thus the mobile phase at the head of the column, loses pressurization.
In addition to interruption of the flow of mobile phase or other fluid components to the chromatographic column while the sample loop is being pressurized, disruption of the mobile phase may further include the mobile phase within the column flowing backwards from the column into the sample loop if the sample loop is not isolated during the pressurization. The insertion of an unpressurized volume into the flow stream feeding the column can cause a negative pressure gradient between the unpressurized sample loop and the highly pressurized head of the column. This negative pressure gradient causes a disruption where the mobile phase can flow out of the head of the column to fill the void created by the unpressurized loop. A backwards flow within the head of the column is considered poor practice as it can lead to failures in the column's packing.
It is therefore desirable to inject sample into a loop, to pressurize the sample loop and to introduce the pressurized sample loop into the mobile phase without disrupting the mobile phase, such as interrupting flow of the mobile phase into the column.